Well, it's important to understand that more often than not, we're making improvements upon compounds which have already been studied rather extensively. In many cases, it's simply a matter of improving pharmacokinetic features (e.g., half-life, oral bioavailability).
In the cases where there are novel compounds, Tim will often contract out a research group to evaluate a given question/concern (e.g., safety, efficacy, etc.).
Often times, I wish Tim would "brag" a bit more about the steps taken for quality assurance and pilot studies evaluating efficacy of new derivatives of certain parent compounds, but he's too busy.
The logic behind supplement development is always long-term. For example, there are a great deal of anabolic steroids we could have come out with, but the new laws make it pretty pointless.
Also, there are often improvements being made to the current line of supplements.
Hope this helps.